Ah, here it's explained clearly:

The Difference Between Retroactive Disability Benefits and Back Payments | Disability Secrets

Back Pay is from when applied to when approved and Retroactive is only for Social Security (SSDI) and has to do with when you became disabled. Since one gets Social Security Disability because someone has paid into the Social Security system through their work, that is the person's money, not the system's.

The 1-year thing makes sense to me, George, (so is probably not right :-) since people might get denied and appeal and laws change so they may be eligible where they weren't before, etc. It is "supplemental" income and not really about one's disability (like SSDI is) but about needing more to live on because one is disabled and cannot work as much. Everyone working/paying taxes has to help pay that bill, there's no automatic money like with Social Security being taken out of one's paycheck for one.